Zen Wood | by Mr.Cline by Mr.Cline

A rather dark, natural, wooden theme with a cool green accent.

Check out Version II as well!
https://addons.mozilla.org/en-US/firefox/addon/zen-wood-ii-by-mr-cline/

You find more natural wooden themes here!
https://addons.mozilla.org/en-US/firefox/collections/15195387/Wood-Textures/